In the prior art, it is well known to join together a pair of preformed parts into a single unitary structure. This joining is particularly prevalent in the automotive industry where a hollow door, hood, deck gate, end gate, trunk lid or the like is formed using these joining techniques. Typically, these doors comprise an outer and an inner panel. The edges of the panels are clinched together using a hemming machine or apparatus.
A widely-used process for hemming of door panels involves a pre-hemming step followed by a final hemming step. In the pre-hemming step, a right-angled flange of one panel is bent over a flat edge of another panel by a pre-hemming die surface. In the final hemming step, the bent flange is flattened onto the flat edge of the other panel to form the hem by another final hemming die surface.
Various types of machines have been proposed to perform these types of hemming operations. One type uses a vertically-driven main die and a horizontally-driven hem gate. The hem gate supports the pre-hemming and final hemming die surfaces and is moved laterally or horizontally for hemming. The main die is raised vertically for the hemming steps. These horizontally-driven gates lack accuracy and repeatability in the hemming process. In these machines, there are typically four separate assemblies to hem each side of a rectangular or square unit. Since each assembly may have its own main die, and drive for the hemmers, the overall apparatus is rather clumsy and bulky.
Another type of hemming machine uses a linkage and a swing-type motion to allow the pre-hemming and final hemming surfaces to contact the flange for hemming. The complicated drive mechanisms associated with these machines make them expensive and can cause unwanted variations over time in hemming performance.
Another hemming apparatus is disclosed in U.S. Pat. No. 5,150,508 to St. Denis. This patent discloses a hemming machine using the horizontally-driven hem gate and vertically-driven main die described above. In St. Denis, the main die is raised hydraulically between two positions for pre-hemming and final hemming. A lifter is used to then remove the hemmed part or load a unit to be hemmed. This machine also suffers from the drawbacks noted above.
In light of the disadvantages of the prior art hemming machines, a need has developed to provide improved hemming apparatus and methods. The present invention solves this need by providing a hemming apparatus having an improved main die drive, an improved hemming die surface drive, and a unique and novel manner in which to position the die surfaces in conjunction with movement of the main die for hemming purposes.